42 



LETTERS ON TREES. 



5. The only difference between that part of the 

 stem which is above ground and that 

 which is below, consists in the absence 

 from the latter (as shewn in fig. 4), of 

 the central medullary cavity. The 

 woody bundles here have still the 

 common cellular tissue for their basis, 

 and the medullary ring and medul- 

 lary rays exist as in that portion of it which is above- 

 ground; but the bundles meet at a point in the centre. 

 There being no buds on this central portion of the root, 

 to be nourished by it (although, as we shall hereafter 

 see, there often is on its exterior, lying underneath 

 the bark, and in connection with the medullary ring), 

 the pith and its containing cavity are wanting. 



6. The disposition of the woody bundles in concen- 

 tric zones or circles, is by no means always so regular 

 as is represented in the foregoing figures. In the Fir, 

 indeed, as associated with other firs in a close planta- 

 tion, and growing chiefly in the vertical direction 

 (the lateral branches or side plants being, from the 

 circumstance of their position, arrested in their 

 growth), the disposition is, on the whole, singularly 

 uniform. In most trees, however, and, in fact, in all 

 that have full freedom of growth allowed them, not 

 only do the zones of different years vary greatly in 

 thickness, but different parts of the zone of the same 

 year are very unequal in this respect, as shewn in 



